Ok, back in September I melted the fresnel on my SB-800. I called up Nikon Repair and they sent me Part SS060-27 Fresnel Lens and SS060-28 Acrylic Panel. The parts were on backorder but they finally got to me. Now the flash did work just fine while melted, no problems at all but the parts cost me less than $20 so what the heck.
First off, Nikon says that these repairs should not be done yourself and should be sent to a Nikon repair facility. This is what I did to save me a bit of cash. This should not be attempted. Ok, that’s my disclaimer. 
I am mechanically inclined and spent years of my childhood taking apart toys and gadgets and years repairing cars.
So here is what it looks like, pretty sad isn’t it? Don’t worry it won’t look like that for long.
As you can see the unmelted flash is on the left and the melted one is on the right. Below is the parts list and diagram for the SB-800 that Nikon was nice enough to include per my request. To be honest, it didn’t help me much, I think it’s missing a few things so it really isn’t all that helpful for me but it might be very helpful to see. Make sure that the batteries are removed first.
Ok, to begin I removed the two screws as shown in the photos, I used a #0 phillips head Craftsman screwdriver .
After the two screws are removed, than remove the rubber pieces on both sides of the flash head and remove 2 screws on the side of the flash head that were on the same side as the 2 that was removed earlier. NOTE: I did not remove all 4 at this time, only 2 of them. (no pictures of that, sorry)
I couldn’t fully remove the lid to the flash head, I have no idea what was holding it together but I was able to make a separation large enough to remove the melted pieces. There is a notch in the housing that the fresnel can fit, it can only properly fit in 1 direction. If it does not close together securely, flip the pieces around. Please see the notch in the 3rd photo below.
During the reinstall, the push button spring and tab will need to be put back in, this is very simple. Just remove all 4 screws on the side, put the spring in, the metal holding tab, and secure it. Following that put the rubber pieces back in, they too can only fit 1 way, look for the notches and they will be just fine.
Then it’s all done. Below is the new side by side shot with the replaced parts and a testing shot of the flash, no problems at all.
Here is it part that melted, they are actually fused together, I couldn’t get them apart. You can also see how much they warped!
Remember, Nikon said do not do this yourself.